The invention is concerned generally with optical beam directing devices and more particularly with a beam director for a spectrograph. Typically, a spectrograph employs a beam chopper to direct the beam alternately through a sample cell and a reference cell. A typical chopper, shown in FIG. 1, employs a pair of stationary mirrors 15 and 16 at opposite corners of a rectangle and a pair of mirrors 12 and 17 mounted perpendicular to rotating shafts at the other two corners. The rotating mirrors 12 and 17 are semicircular so that during half of each rotation, the mirrors direct a beam 11 along a path 13 through a sample cell 133 at the top of the rectangle and for the other half of each rotation, the beam is directed along a path 14 through a reference cell 134 on the bottom of the rectangle. In some devices, a number of sample cells can be successively rotated into the spectrograph beam by a carousel. Vibrations from the mechanical motion of the mirrors and carousel or misalignment of the sample cells can generate small deviations in the beam direction which limit the accuracy of the spectrograph. Since the spectrograph's grating produces an image of the slit on a detector array, a detector in the array must be as large as the image. To accomodate a large number of detectors, it is necessary to use a narrow slit and narrow beam. The spectrograph is thus sensitive to deviations in beam direction so that the spectrograph accuracy can be increased by use of a beam directing system that adjusts for deviations in beam direction.